Many works have exploited wireless signals, for example, FM radio [1]}, [2]}, RFID [3]}, [4]}, acoustic [5]}, WiFi [6]}, [7]}, and BLE [8]}, [9]} for fingerprinting.
Among them, WiFi access points are the popular choice since they are readily available in many public locations; whereas the ubiquity of Bluetooth technologies introducing new RSS-based fingerprinting with BLE beacons.
Other wireless technologies, on the other hand, require specific infrastructures and dedicated devices, which are not always available to end-users.
Since our work focuses on location estimation with smartphones, this section mainly reviews the wireless signals (i.e., WiFi and Bluetooth) that are familiar to end-users.
